The present invention relates to an alignment method and apparatus for a reduction projection type aligner in a reduction projection exposure process wherein the coarse detection of reticle position in reticle alignment and the fine detection of reticle position in wafer alignment are effected by use of the same reticle alignment pattern and the same optical alignment detection system.
With the progress of miniaturization of semiconductor integrated circuits, demand is for higher accurate alignment between reticle and wafer in the exposure with reduction projection type aligner. In view of this, it is apparent that the TTL (Through The Lens) alignment method with a reduction projection lens which is capable of meeting a chip arrangement error in a wafer by chip-by-chip alignment will account for a majority of production systems for high-density integrated circuits in the future.
In a conventional TTL alignment method as shown in FIG. 1, for example, each position of reticle initialization patterns 15, 15' is detected by optical reticle alignment systems 5, 5' to set a reticle 1 at initial position. The illumination light of the same wavelength as the exposure light from optical fibers 11, 11' of optical wafer alignment detection systems 6, 6' are applied through half mirrors 7c, 7c', condenser lenses 7b, 7b' and mirrors 7a, 7a', radiated on reticle alignment patterns 13, 13' separate from the reticle initialization patterns 15, 15', and then radiated on wafer alignment patterns 14, 14' through the reduction projection lens 2. The reflection light from wafer alignment patterns 14, 14' form images of wafer alignment patterns 14, 14' on the alignment patterns 13, 13' on the reticle 1 through the reduction projection lens 2. The patterns 13, 13'; 14, 14' are passed through the mirrors 7a, 7a', condenser lenses 7b, 7b', half mirrors 7c, 7c'magnification lenses 8, 8', mirrors 7d, 7d', and movable slits 9, 9' to photomultipliers 10, 10' to detect a position. If the positions of the patterns 13, 13' and 14, 14' fail to coincide with each other, a wafer stage 4 carrying the wafer 3 is moved along x and y axes thereby to render the positions of the patterns 13, 13' and 14, 14' coincident with each other. Upon completion of alignment this way, the exposure light from the exposure system 12 is used to expose the circuit pattern of the reticle 1 through the reduction projection lens 2 on one to several chips on the wafer 3. The alignment method of this type is disclosed in Japanese Patent Unexamined Publication Nos. 144270/78, 93974/79 and No. 41739/80.
In this alignment method, four optical alignment systems including the optical reticle alignment systems 5, 5' and the optical wafer alignment detection systems 6, 6' are arranged around the reticle 1, thereby posing the problem of a complicated and bulky general configuration. Another disadvantage is that a large detection field of view of the optical reticle alignment systems 5, 5' is required with the ordinary mounting accuracy of .+-.1 to 2 mm for the reticle 1, resulting in a lower detection resolution and a lower detection accuracy. Further, in the production of a high-density integrated circuit, the use of a resist is being studied with a light absorber or a multilayer resist to cope with the problem of defocus caused by the roughness of the resist surface or multiple interference in the resist. Since these resists offer a very low transmittance to light having the same wavelength as the exposure light, however, it is impossible to use the light of the same wavelength as the exposure light for illumination of a wafer alignment pattern. As a result, the exposure light and the light for illumination of a wafer alignment pattern are required to have different wavelength. Nevertheless, in view of the fact that the chromatic aberration of the reduction projection lens 2 is corrected only for the exposure light, the image of the wafer alignment patterns 14, 14' is not formed on the reticle 1 any longer but at a position distant from the reticle 1 by the amount of the chromatic aberration. Therefore, it is necessary to detect the reticle alignment patterns 13, 13' and wafer alignment patterns 14, 14' separately from each other, thereby further complicating the configuration of the optical detection system. In addition, the position of the reticle 1 is required to be detected twice during the period from the mounting of the reticle 1 to the complete detection of the wafer alignment, thus reducing the throughput.